可见光响应光催化剂K4Ce2Ta10O30、K4Ce2Nb10O30及其固溶体的电子结构

基于密度泛函理论(DFT), 采用平面波赝势(PWP)以及广义梯度近似(GGA)方法, 对可见光响应的光催化剂K4Ce2Ta10O30、K4Ce2Nb10O30及其固溶体进行电子结构的第一性原理计算. 结果表明, 光催化剂K4Ce2Ta10O30和K4Ce2Nb10O30的导带分别主要由Ta 5d和Nb 4d组成, 处于高能级的电子未占据态的Ce 4f与其有很明显的重迭, 但由于其高度局域特性,不能很好地参与光生电子在导带中的传导, 从而对光催化活性的贡献很小;而其价带则由O 2p与Ta 5d (Nb 4d)的杂化轨道组成, 同时电子占据态的Ce 4f对价带也有一定的贡献, 各个电子轨道对能带结构的贡献决定了该系列可见光响应光催化剂的物理化学和光催化特性. 固溶体系列中随着Nb含量的增加, 其吸收光谱依次红移, 带隙变窄, 导带底变低, 光生电子的还原能力降低. 在固溶体K4Ce2Ta10-xNbxO30(x=2, 5, 8)中, 由于Ce 4f轨道对价带顶的贡献相对较小, 固溶体的价带顶低于K4Ce2Ta10O30、K4Ce2Nb10O30的价带顶, 光生空穴的氧化能力相对较强. 该系列光催化剂的电子结构分析结果与光催化水分解的活性实验结果有很好的一致性.

Electronic Structures of the Visible-light Driven Photocatalysts K4Ce2Ta10O30,K4Ce2Nb10O30 and Their Solid Solution Compounds

First principle calculation was carried out to study the electronic structures of photocatalysts K_4Ce_2Ta_(10)O_(30), K_4Ce_2Ta_(10)O_(30), and their solid solution compounds, which were all capable of responding to visible light, based on density functional theory (DFT) within plane-wave pesudopotential (PWP) and generalized gradient approximation (GGA) method. The calculation results indicated that conduction bands of K_4Ce_2Ta_(10)O_(30) and K_4Ce_2Ta_(10)O_(30) were mainly attributable to the Ta 5d and Nb 4d orbitals, respectively. Although the unoccupied Ce 4f orbitals had overlapped at the bottom of the conduction band, they were less effective in transferring electrons and photocatalytic activities for their high localized nature, while their valence bands were composed of hybridization with the bonding of O 2p Ta 5d (or Nb 4d) and occupied Ce 4f orbitals. The contribution of these orbitals to the energy bands affected the electronic structure of photocatalysts and gave rise to their differences in light absorption and photocatalytic activities. As for the solid solutions compounds of K_4Ce_2Ta_(10-x)Nb_xO_(30)(x=2, 5, 8), their band gaps decreased with an increase of the x value resulting in a corresponding lower reducing ability to evolve H2. The valence band edges in solid solution compounds were lower than that of K_4Ce_2Ta_(10)O_(30) and K_4Ce_2Ta_(10)O_(30) for the little contribution from occupied Ce 4f. This consequently led to the higher oxidize ability. The results of the analysis of electronic structures were in good accordance with their experimental photocatalytic activities for water decomposition.